Magic filters slowly through the bloodlines of a species. When the deer began at first to harness the arcane energies that filled the space about them, the resulting spells and forces were extremely weak. A deer, even one so mighty as a strongly-antlered elk, would have to strain merely to move one rock from one pile to the next. But magic, at least the magic of My Little Pony: Friendship is Magic, works rather like a muscle. The more you practice it, the stronger it becomes.
However, it is more precise to state that the magic of the ponyverse acts like a muscle if its gains of strength switched off from being temporal to being most genetic. When a pony- or a deer, or an antelope, or any such creature- has been working with their magic for some time, that magic is much amplified inside them. And then that amplification is impressed upon the very blood and marrow of the creature casting spells. This means that nearly every gain in magic power born from careful effort becomes permanent- and even more than that, becomes inheritable. The very first elk, red deer, mule deer and more who used magic built upon their pitiful abilities through careful effort- and the strength they gained they passed along to all deer that came after.
Now? The deerfolk, and the Elders that are their most mighty remnant, represent the apex of the deer's long bond with magic. Fëanor and all his fellow high deer Elders have within their veins the magic of millennia of effort. This has made them mighty far beyond compare, with spells of such a scale that they might shatter mountain ranges and set all the seas to boil. The common deer- the mule deer and the white-tails- are not as strong, but they too on average have far, far more magic than a pony unicorn.
And magic is the strength of life within the ponyverse, which means that once a creature passes one specific level in their arcane strength they turn effectively immortal. Morta, Decima, Nona and the other elders have persisted young and strong for ages- the youngest of them, Glorfindel, is twenty-five hundred years old and still counting. Mule deer and white-tail deer live centuries.
Far beyond what any pony might achieve? Not quite. The ponies are a very young race, their bond with magic less than two thousand years in age. They have, in contrast to the deerfolk, only just begun to tap their magical potential, and the gains of all their elders have already been imprinted on the newest generations. Twilight Sparkle is of special note. Even as a 'normal' unicorn she wielded power stronger than a common deer, and her possession of the Element of Magic cannot be the only explanation. I suspect she comes from some old line of sorcerers, perhaps from deep within Equestria's more primeval past, and the strides they made in magic were transmitted through the years to her. Were Twilight to birth foals, and those foals in the future to have children, eventually a line would rise of unicorns as powerful as high deer. Twilight in her strength is a firm glimpse into the future of the ponyfolk's arcane potential.
Additionally, ponies have one more trick up their sleeve. The high deer are immensely powerful, comparable to gods. But they are not gods- yet Celestia and Luna are. The coming of the Sisters changed the nature of the pony species, giving them, in essence, a long line of golden thread that stretches up to heaven. Sun and Moon have ties to the infinity of all the cosmos, and through them, other ponies have the chance, however rare, to reach godhood themselves. Princess Cadence is the first of this example, and Twilight herself is now the second. Might other ponies, somewhere further down the line, achieve divinity? Time alone will tell.